In an announcement that will shock the theatre world, Jonathan Kent and Ian McDiarmid have revealed that they are to resign as directors of London's Almeida Theatre. They plan to depart in July 2002. But their legacy will be formidable. Since 1990, when they took over the Almeida, they have turned it into a world class theatre. Their successor, as yet unknown, will also inherit a building that has been handsomely refurbished, with the help of lottery funding, at a cost of £5.8m.

So why go? Kent and McDiarmid are currently running a successful season in a converted King's Cross bus station, from which Neil LaBute's The Shape of Things is about to transfer to New York. Productions of King Lear, Chekhov's Platonov and Brian Friel's Faith Healer are forthcoming. The funding situation is also about to ease: a grant of £498,950 in 2000/01 will go up to £975,561 by 2003/04. And, on top of all that, the money is pretty much in place to carry out the renovation of their home base. Isn't it a funny time to leave?

"No," says Kent, who looks pretty drained after a 12-hour day working on Platonov. "I think the time has come. It's been in some ways a struggle to create the Almeida. It's also been the most remarkable time in my professional life. But after 12 years, one's energy and initiative start to flag. Also, given the rise in funding, the building looks as secure as it's ever been. So it's probably time for a fresh vision. The Almeida, unlike the National, is an act of imagination. The danger, if we stay on, is that we become an institution."

Mention of the National prompts speculation that they might have the directorship of that building in their sights. "Absolutely not," says McDiarmid. "There's no hidden agenda. We're not moving on to something else. In fact, one of the things I like about the theatre is that one thing doesn't necessarily lead to another. Neither Jonathan nor I are careerists. In a sense, the Almeida ambushed us. We'd both worked there as actors under Pierre Audi, and he suddenly asked us one day if we'd ever thought of running a theatre."

McDiarmid was initially reluctant to have his energy used up on the endless fundraising that is so often the task of the theatre director. But running the Almeida eventually brought its rewards. A series of successes attracted the likes of Kevin Spacey, Juliette Binoche, Ralph Fiennes and writers Pinter and Hare. Nevertheless, the business of running the Almeida remained a perilous and demanding task for the directors.

"We've run a National Theatre repertoire on a twentieth of their budget," says Kent. And McDiarmid confirms how close the pair have sailed to the wind. "We initially announced a season we didn't have the money for. And even when we opened with Howard Barker's Scenes From an Execution, I could never quite believe that its star, Glenda Jackson, would turn up. She was in Los Angeles in a production of Who's Afraid of Virginia Woolf? that had overrun and she offered us the chance to replace her. Of course we refused. And there she suddenly was one day at the Almeida, in her red raincoat, helping to get the show on in two-and-a-half weeks. But we've lived through a series of happy accidents. When we converted the Gainsborough Studios to do Richard II and Coriolanus, we had to raise a million pounds in six weeks. The board thought we were mad and we had to threaten to resign to convince them it could work."

Kent jibs a bit at the phrase "happy accidents". He sees the Almeida's development as a series of planned metamorphoses. "One possibility was doing Richard II and Coriolanus at a dog track: I'd love to have seen that on a CV," he says. "And, when we had to move out of the Almeida, I originally planned to open a new theatre in King's Cross with Bernstein's On the Town. I wanted the sets to be moving so that the city appeared to be coming towards the audience. Theatre has now moved to the margins of culture, allowing for endless possibilities."

So what happens next? Both have a busy schedule. Kent will direct David Hare's distilled translation of Chekhov's Platonov and, in quick succession, Faith Healer and King Lear with Oliver Ford-Davies. McDiarmid will act in the Friel and is also in a new BBC Crime and Punishment. "Inevitably our careers will diverge," says Kent. "Ian acts. I direct."

It would be a tragedy for the British theatre if Kent and McDiarmid, having turned the Almeida into a powerhouse, did not continue their work elsewhere.

When I ask about unfulfilled dreams, McDiarmid produces a comprehensive list from his briefcase. He is nervous about revealing it for fear others may get in first. But it includes Ibsen's massive and virtually never staged Emperor and Galilean, Racine's Berenice and Athalie, Goethe's Faust Part One and Bond's Lear. Kent chimes in with his own list, including The Odyssey, Pericles and, of course, On the Town. "Leaving the Almeida is not about diminution of appetite," he says. "I'd love to do plays at the end of a pier." The mind boggles. Emperor and Galilean in Bournemouth? "More Cromer, perhaps," jokes McDiarmid.

It was precisely the adventure and irony that made Kent and McDiarmid, a dauntless South African and a foxy Scot, such a formidable artistic team. Whoever succeeds them will have a hard act to follow. "The British theatre's oldest phrase was always, 'That's impossible.' No one ever dares say that at the Almeida. People now say, 'There must be a way.'"